Ethylene glycol, also known as glycol or ethanediol, is a commodity chemical and can be used to produce polyester, polyethylene terephthalate, explosives, solvents, antifreezes, plasticizers, and hygroscopic agents. Due to the confinement of technical monopoly and limited oil production, the supply of ethylene glycol in China has depended on the import for a long time. Since 2002, the import of ethylene glycol of China has accounted for >70% of the quantity demanded wherein the consumption of ethylene glycol in polyester industry contributes >80%.
The conventional syntheses of ethylene glycol are based on the petrochemical industry, which mainly use ethane as the raw material to produce ethylene glycol through the addition reaction between the intermediate of ethylene oxide and water. Simultaneously, by-products of diethylene glycol, triethylene glycol, and other low-value chemicals are formed. The key techniques are mainly possessed by Shell (the United Kingdom/Netherlands), Halcon-SD (the United States), Dow Chemical (the United States), and UCC (the United States). In addition, the processes of hydration and separation consume a large amount of water, energy, petroleum and natural gas. A pathway through the intermediate of ethylene carbonate is a novel synthesis method of ethylene glycol, which takes advantage of the highly pure carbon dioxide discharged by the unit of production of ethylene oxide. This method can reduce the emission of carbon, and barely consumes water, which also reduces the consumption of energy in separation. A more marked merit of this method is the co-production of dimethyl carbonate. However, this synthesis pathway also consumes ethylene oxide, still depends on petroleum, and the key techniques are also kept by several companies.
With the progress of science and technology, coal- or biomass-based syntheses of ethylene glycol have shown many advantages gradually, such as reducing the dependence on petroleum and breakup of the technical monopoly. Preferably, the biomass-based synthesis pathway primarily utilizes low-cost, fast-growing, and carbon-neutral cellulose-enriched xylophyta or herbs as its feedstock. Hence, both the costs and greenhouse effect of the production of ethylene glycol are effectively reduced. Thus, this biomass-based synthesis pathway is a promising technology.
Coal-based ethylene glycol can be synthesized via the electrochemical hydrogenation and dimerization of formaldehyde, polymerization of methanol, or hydrogenation of oxalate. The yield of ethylene glycol through the electrochemical hydrogenation and dimerization of formaldehyde is high, while the electricity consumption is high and purity of raw ethylene glycol is low, resulting in high costs of purification. The method via the hydrogenation of oxalate is the most promising synthesis pathway of coal-based ethylene glycol, which utilizes CO in the syngas as the intermediate to produce oxalate through oxidation-coupling reactions and then synthesize ethylene glycol through the catalytic hydrogenation of oxalate. On account of excessive hydrogenation, part of the system is converted into methanol and ethanol, which will be reacted with ethylene glycol to form 1,2-propanediol and 1,2-butanediol respectively via the Guerbet reaction. Wherein, the boiling points of 1,2-butanediol and ethylene glycol are the closest, and both diols can also form an azeotrope. Therefore, the energy consumption of distillation in the industry is high, which is an important reason for the difficult industrialization of coal-based synthesis of ethylene glycol.
Because of the limited selectivity of catalysts for the biomass-based synthesis of ethylene glycol, during the hydrogenation process, apart from the main product of ethylene glycol, miscellaneous diols and triols at a total yield of ˜30 wt % to 40 wt % are generated, such as 1,2-propanediol, 1,2-butanediol, 1,3-butanediol, 2,3-butanediol, glycerin, and so on, wherein these miscellaneous diols have a boiling point higher or lower than that of ethylene glycol by ˜10° C. and certain diols can form an azeotrope with ethylene glycol, resulting in the significant decrease of separation efficiency by distillation and increase of energy consumption. The physical and chemical properties of these miscellaneous diols are similar to those of ethylene glycol, and hence it is difficult to separate them via conventional techniques including distillation, extraction, adsorption, etc. In addition, the viscosity of the mixture of these polyols is high at room or low temperatures. Especially within narrow space, the flow resistance will be great, increasing the energy consumption of transportation. Meanwhile, the expected output of ethylene glycol, as a commodity chemical, is high while the processing capacity of liquid-phase adsorption is low. Therefore, adsorptive separation is also not applicable to the separation of these polyols.